Radio angle reception meter



Aug. 5, 1952 Filed May 25, 1950 J. T. DE BETTENCOURT RADIO ANGLE RECEPTION METER E 2' SHEETS-SHEET 2 0./ v o. u i lk ANGLE-A /A/ DEGREES Patented Aug. 5, 1952 Y UNITED STlli'IES 2,606,319 p f f l aspro' ANeLnennoEraioN-rritirna .oseplo T. degettencourt, Ghestnut-HillfMassl, assigner to Raytheon ll/ianulfacturingftloxnpany,v Y Newton, Mass., a corporation-of Delaware` application May 23, lesa-serial m9163345@ This invention relates to 'measurement of" the angle of arrival of 4pulsedra'dio 'en'ergytransrnisf sions.

The irieasurementof4 the angle" of' arrivalof v radio energy Waves is of importance particularly in the study of Wave propagation. For example,

radio energy from a transmitter at the surface of the earthmay reach a receiver at aselected pointY by several possible paths or modesof propagation some of which', particularly in the case of very long distances between transmitter and receivery are dueto Vreflections from the .E and F layers kof the-ionosphere. These-reections, in arriving.V vat a receiver near-thelgroundVare directed-clovvn Wardly; andare hereinafter termed down-coming these down-coming Waves from the earthissurface arriving at the receiver and-hereinafter termed reiiected Waves., In the-studyf-of these' various angular paths of arrivalfrom the-ionojsphere and the yearths reflections, it-becomes-de-l ..15 radio waves as-distinguished romfrelections'of l sirablel-to isolate selected modeslof propagation at the receivedV SuchA isolation :makes-possiblew` a clear indication of the angle of arrival due tothe4 Y selected-mode andassists in a 'betterfunderstande ing or" the propagation Yof radiowavesf Pursuant to thepresentinvention, isolation inf aradioeenergyfreceiverto asingle mode of prop` .agation or to alselected group of modes of'prop--` agation may. be.l obtained and angleffoarrival' readings-of the down-coming vradio Waves-'of such selected mode'of propagation mayb'e vaccurately and rapidly .1n-ader In the present invention radio energy. pulses` are emitted-,from atransmitter at suitable intervals;

The :receiver which-is tuned to receive the energy'.l

oithese pulses incorporates fa gatingarrangement synchronized with thefpulses from thetransmitter and controllable as tothe time in whichthegating'.

periodbegins .with respect to the emitted 'pulses' and as to the duration sof. the gatingrperiodi A` transmitted pulse will appearIatl'thelreceiver as I a trainof pulsesrdue tonthe ivariatio'n A`inf-travel' time determined 'bywtnev various modes l' of 'pro'p agati'dn.A Thus, bythe :above gating arrangement f receiver: for. purposes of J study.

Such agating arrangement; however-o itself-is notsuc'ient to satisfactorily isolate a mode-:ofi

propagation.' During the selected-arrival pulsein addition-1to1 the down-coming'radiciV waves, lthe above-mentioned relections from the earths surface also occur the receiver. It isjrthereforenecessary to provide an arrangement whereby the angle of arrival of the down-coming'iaves may be determined despite tlese'gro'uii'drr'eletions.l For this purpose, a triple antennamarray, to be herein after more fully-describedfis usedl- The combina-' tion xofftlie rabovemeritiened gating arrangementwith the triple antennaY array-thereby A1x1-altes''pofsl- A sible obtaining- -in-forination pfff-tilel receiver Ifor measuring the Y'angle VOYa'rrival of doWn-com'in'gf radio Waves o`a'se`l'e'cted inode ofpfOnagai'L'n.vlv i Such angular information is,obtainedi'n-"the receiver generally as follows. Thethree' antennas inthe array'are arranged horizontally inaplane" perpendicular tothe-.groundand with-the; two outer antennas in one circuit and the .middle ar.-n tenna in a second circuit. Voltage signals induced inth'e` antennasthiprpagatedadio Waves appear in thetvvo antenna Ioirciiit and are applied 'to Yadding A'and"subtracting; preamplifier circuits. Theseadded and subtracted voltage signals are each sent to a superheterodyne receiver having a common heterodyning oscillator where they are suitably amplified-.f Tli'e rectified output signals from the superheterodyne receivers are each" sent to a'g'a'tedi'r'eak"readingy rectije'r ciritfj The gating'of 'the' peak'rea'ding rec'tier' is achieved by twojmnstable mlltivibrat, o'e arranged to set the starting position `of 'thegateM `and the other arranged to set the Width ofthe;

gate. A trigger generator producing#pulsesl synchronism withthe-pulses ofthe transmitter is coupled Avto vthe gate positioning multivibrator. The signals from thewtwo peaking circuits are then comparedin an electronic vcomparisor'i or ratio circuit. The resulting comparison is then""co'n"" tinuously registeredv on anlelectronic strip chart recorder for readyvreference in determining the angle of arrivalV ofthe radican/es of trie selected- Inode ofpropagationi" y g' The above kand othrratnfefs',fob5e`t1a adi' vantages fof nie' invent'inlwillj b'come more api-4` parent "from Ythe following descriptionI taken-in .connection with the accompanying drawing-siffvvhereintfV Fig. l visi'Y a r partially*v schematicandrpartiallyi block diagram offa suitable gatedreceivinglappl-'if'- ratus for use in the invention`;'l I

Fig. 2 is a View illustratingsome representative paths of travel of `'radio energy Waves from the transmitters to'- the receiving apparatus Afin-'tide invention;

Fig. 3 is a diagrammatic View of a triple an- :tenna array showing down-coming and reflected Waves for aid in illustrating the operation of the a down-coming radio Wave I8 regardless of the simultaneous existence of a radio wave reected from the surface of the earth I6. Designating a horizontal component of the downcoming radio wave I8 at the antenna I2 by ed and that of the reflected Wave 20 at the antenna I 2 by en the instantaneous values of ed and en may be be expressed Y in' which Ed and ER are'the maximum amplitudes of horizontal components of the electric elds (at antenna I2) of the down-coming and reected wavesfwis the angular frequency of the Waves, t the time and bis the phase diierence between the two horizontal components arriving at antenna I2.

Assuming the plane of 4incidence of vthe wave I8 is perpendicular to the plane of the antennas then the instantaneous values of the horizontal components of the total elds at antennas I9, I2andlflaie`-A `e14=Ed COS (go-qQ-I-Erz COS (mV-hp) inwhich 1p' is determined from Fig. 3 as v bsin A where d and y v(the'wave length ofthe radio wave)' are measured in the same units.

'By connecting antennas I0 and I li the combined'voltage induced in them produces a voltage givenby The Vfield at antenna I2 produces a voltage In the above 'equations the constants K1 and K2 are system constants depending upon the antenna size, spacing d, height above the earths surface I6 and the constants of transmission line and couplingto suitable utilizing*V circuits suchv Also, V1 and V2 are subtracted'by an antiphase circuit to give a voltage remi-.vv

In the embodiment in Fig. l, adjustment is made so that K2=2K1 for which condition the ratio R becomes 1 -M 2 e 1-poos .1a-tan 2 and the ratio Ro is then measured. From this measurement of Ro and the theoretical discussions above, suitable charts may be arranged for directly finding 1p and the angle A, the angle of arrival of the down-coming radio waves. One such exemplary chart is shown in Fig. 4.

It is noted that for a given value of Ro increases with the angle A. Also, the accuracy is greater for lower angles of arrival.' The low angles of arrival are generally of'greatest interest inapplications to long distance transmission. 1

The above analysis of a single mode of propagation may also be extended to multiple modes of propagation. Because of the gating arrangement for isolating a mode of propagation in the embodiment in Fig. 1, this analysis is directly applicable there.

Referring now to Fig. 1, antennas I, I2 and Ill, in this instance dipoles, are arranged as explained -with regard to Fig. 3. The outer antennas I 0 and Ie are connected by lines 22 and 24, respectively, across primaries 26 and 28 of radio frequency transformers 30 and 32 in the voltage adding circuit 34 and voltage subtracting circuit 36, respectively. The antenna I2 is connected through lines 38 and 6D across the primaries 42 and 44 of radio frequency transformers 46 and 48, also in the voltage adding and subtracting circuits lSil and 36, respectively. The l radio frequency transformers 30 and 32, 46 and 48 are preferably provided with electrostatic shields 56 and 52 between primaries and secondaries and suitable inductive electrostatic shields 54 and 56 between transformers 30 and 46, and 32 and d8, respectively. In the voltage adding circuit 34 variable capacitances 58 and 60 are connected across the secondaries of transformers 36 and Q6, respectively, to provide resonant circuits tuned to the radio frequency of the received signal pulses. These tuned circuits provide the input to the'control grids of amplier tubes 62 and 64, respectively, which may be of the 6SK7 type. The amplifier tubes 62 and 64 are ccnnected in` parallel across an inductive capacitive tank circuit 66 tuned by means of a variable condenser 68 to the input signal frequency. The gain -of tubes 62 and 64 is controlled by variable resistances 'ID and '12, respectively, and provides the K1 and K2 adjustments hereinbefore mentioned. Roughly, the adjustments of resistances I0 and 'I2 for making K1=2K2 are approximately at a point where the gain of tube 64 is twice that of tube 62. Accurate adjustment may be made by linitially adjusting to a controlled input signal of known value. A

The. voltage. signalsinduced. in antennas i;

andf 1.4 and antenna l2. appearA through.. trans.- formers. 30. and. etanol; are. amplied by. .tubes 62.-. and. 6.4, respectively, and the. added Composite. 0f. the. two. voltages.. will appear across the reso.- nant.. circuit. 6.6.., The. inductance Coil: ofl the resonant.. circuit 5.5.15. coupledA Vt0 a Secondary 0.01.1v 14.. by. means. O f. which the added voltage. Signals.

are-.made t0. apnea-.r through line. T6. at a. Suitable superheterodxne. receiver 1.8, whose .hatarodvning frequency obtained-through1ine 8.8. from an.

Oscillator .8.2.` The. lsonerbeten.dyn.e. receiver. T8 is. Qt. conventional .tyre .Suitable for. amplifying pulsed signalsu of the particular frequency used. The detected output.. at. the receiver la appears throuehlirlat and ampliati@ by. an' amplifier tutti. tc. falsa the receiver Output Signal in raagingrectiiiar circuit. S8.- 'lhe peak readgretaa circuit what a capacitor 9@ having 1-o @page d. t9. the Cathode. 0f the am- Diier' tube 8a and. a. grounded resistance 92 cnn'cteduthrough lineV 9 4 to the other side of capacitance 90, L4A tri'de 9c' has its anodecon- 'nec'ted'tc line Silfbetvvwen capacitance 90 and resistance 025v and its cathodeconnectedto ground. GidQ 9 8. ofA triode 95 is connected to a line |00 vleafdijngfffrom af gating circuit |62, tok be hereintto. aaoutput level convenient for a gatedv tothe logarithm oftheinput, A Sutablaresist:

throfu'gl'iv resistance. 9dA which is chosen at such a valu olaslto, produce along time constant (l2 secondsfin lthe"present embodiment) Vcompared to' the repetitive period Vof the gating pulses H04 (,Qfsecond in this instance). The peak voltage in the peaking circuit 8.8 1will thereforey continuouslyfa Vpear throughline te at grid Hit of triode'|0i 1nthe comparison or ratio meter circuit H0.

"he voltage subtracting circuit 3B. is similar to theV adding circuit 34, except 'for the parallel connection of the preamplifier tubes across the output tank circuit. Amplifier tubes ||2 and H4 in the voltage subtracting circuit 36 are connected in push-pull across a tank circuit ||6 consisting of an inductance` ||8 and a split stator type 'variable condenser |20 having the third plate |22 connected to the midpoint of the inductanceu i8 Aand the source of positive potential |24,r such as a battery. rVariable resistances |26 and |28l Ycontrol the gain of amplier tubes ||2 and ||4, respectively', and provide the setting for Ki and K2 as explained above.

A sup'erheterodyne receiver |30, which may be similar to the receiver 18 and receiving its heterodyning frequency from oscillator 92, is coupled to'the inductance coil H8 of the tank circuitV IIB by-an inductance coil i32. Thus the preamplied voltage difference signal from the voltage` subtraCtingcircuit 36 appears through line I34"at the receiver |30 where itis suitably amplied and detected. The amplified and detected vl'tagesignal appears Ain the output line |36 and is further amplied to a suitable level by an amplifier tube |38 fora voltage peaking circuit |407. Amplifier tube |38 may be similar to amplifier tubeG, and the `voltage peakingcircuit |40v may besimilansto the voltage peakingcircuit 88,1 and.v

similarly.` havns'- a lone. discharge; time..

capacitor |42 and resistanoellt, and atriodo. l.. 5

with. a srdlacontrollede byv gatingv pulse.. |04. for gating the peaking. circuit |48 similarlvato. thegatng in peaking cruitt- Tous the Realivoltage. determinedy by the.. Signal in theoutput line, 3 6"* and amplifier tubo. |38 will appear. through line |50 at grid` |'52;of triade.I 541inthe comparator or. ratio circuit H0.

The. comparatoror. ratio circuit Il.0.:,has.two.-.

triodes- |08 and litconnected in parallel; irontr the positive. potential line |56. to ground. TheV potentialv across trodes |08. and |54. duev `to..t1 1e positive potential in` line, |;5 fromfr a, potentiali source; such asbattery. |58; and thenormakbiasf. e ing potential-from linesV Stand |50 at gridslfl 6;.

and; |52is such that the triodesvl and [5.4; are` normallyV conductive,

lines Ill-and |74, respectively, to a` ratio voltage recorder |16. The resistance: networks maand |10 have preferably voltage outputs proportional ance network of this type s-,commerciallylnown as Logatens and maybefobtainedcommercially fromY the Kalbfell Laboratories, San Diego, Gali-s fornia. lt should be understood here thatpther logarithmic networks, such forvexarnple. asemploying logarithmic diodes or variable,transcon-l ductance tubes, may also be used, The voltages... inline |12 andA Haare initiallybalancedfor noY signal voltage condition by. suitable. setting of.; adjusting arm |78 on areference-voltagedivider resistance |80 across which. apotentialy gradient appears; due to potential source I,8Z..S.uch as 'a battery. Y

Representing the potential at grid.; Irl'iA by Ei. the potential at` grid |52 by. E2 and thefvoltage difference betweenpoints |614 .and I SQby-EQ, then Due to the logarithmic resistive" networks |6VB,.

and.. H0. the. potential." (liderar1ct.L between. lines. |72 and H4.. thereby is, proportional.. to. 10g. Eli-:log Eiv or @a v. Ezi.

which isrecordedcontinuously by recorder |'|6. .A recorder suitable.. for use. as recorder.. |16 is known asV Brown Electronik Strip Chart Recorder. The recorder may be directly calibrated, therefore, in terms of the ratio Ro which can `then be used for obtaining angle of arrival by means ofA a chart, such as shown inrFig. 4, orY suitable mathematical computation, or the recorder maybe directly calibrated in terms of theangle of arrival.

In the gating circuit` |02; two Inonostable multivibrators |84 and ISG'are used. In the mono- Adjusting arm. L69 is set.Y at a.; position onV resistance |,62, in.the. anodecir- I conductive.

7,., stable multivibrator |84, triode |88 is maintained normally conductive and triode |99 is maintained normally non-conductive.v In the monostable multivibrator |86, triode |92 is maintained normally conductive and triode |94 Vis maintained normally'non-conductive. The triode |96 is also normally conductive thereby maintaining a sufciently negative potential in line |99 to bias grids 98 and |48 below cutoff so that triodesv 96 and |46 are normally non-conductive. When a negativiepulse |98 appears in line-299 and thereby through condenser 292at grid 294, it causes triode |887to-be extinguished. vWhen triode|88 is extinguished, the potential inline 295- lead-v ing'from its anode rises thereby causing arise in .potential at grid 298 sol as to make tube' |99 Conduction in tube |99 causes a drop in potentialV inl its anode line '2 I9 which appears through condenser 2 |2 at grid 294th`ereby maintaining a bias below cutoff at grid 294. The condenser 2|2 immediately begins to discharge through resistances 2 |4, 2|6 and that portion-of resistance 2|8 determined by the setting of adjusting arm 229. When capacitor 2|2 has discharged to the grid base potential of triode |88, the triode |88' will again begin to conduct. When thetriode |88 again startsto conduct, thepotential in its anode line 296 drops causing a corresponding drop in line 222 and through capacitance 224 at grid 226 in the triode |92. AThis drop in potential at grid 226 makes triode |92 nonconductive. When triode |92 becomes non-conductive, the potential in its anode line 228 rises causing a corresponding rise through capacitance 239 at grid 232 so as to make triode |94 conductive. AThe resulting drop in potential in linerill in the anode circuit of triode 94 appears through capacitances 236'at grid 238 so as to' cause triode |96 to become non-conductive thereby resulting in a rise in potential in line |99. This rise in potential appears as leading edge 249 of the posi-l tive gating pulse |94. When triode |94 became conductive, the potential drop-in its anode vline 234 appeared through capacitor 242 at grid 226 so as to maintain triode |92 non-conductive. The capacitor 242 immediately begins to discharge through resistances 244, 249 and that portion of resistance 248 determined by the setting of adjusting arm 259. When the'discharge of capacitor 242 causes a rise in potential at grid 226 to the grid base potential, triode |92 will again become conductive' thereby extinguishing triode |94. The extinguishing of conduction in triode |94 causes a rise in potential in line 234 and thereby at grid 238 so that triode |95 becomes conductive and causes a drop in potential in line |99 appearing as the trailing edge 252 of gating pulse |04. lItvshould be noted that the position in time of the leading edge 249 with respect to the negativev trigger pulse |98 may be -controlled by proper adjustment of the adjusting arm 220 on resistance 2 I8. Also the position of the trailing edge 252.and thereby the duration of pulse |94 may be controlled by proper adjustment of adjusting arm 259 on .resistance 248.

In the present embodiment, the negative trigger pulse |98 is produced b-y a synchronizing pulse timer 254 which may consist of a'conventional variable frequency oscillator having a "manual control 256 for varying the Afrequency of oscillation and a suitable pulseformer for converting Thus by proper pulse from the transmitter used in the system and thereby provide synchronous timing for each gating pulse |94. The frequencyy at which the pulse timer 254 is synchronized with the transmitter may be recognized by inspecting the recorder |'|6 during adjustment of the manual control 256. The indication on recorder |16 will not be steady until the trigger pulse |98 rate is the same as the rate of radio energy pulses from the transmitter. The reason for this is that only at the synchronous frequency does a particular mode of propagation become isolated so as to produce an indication at recorder |76 of only the isolated mode.

Referring to Fig. 2 wherein is shown an exemplary operating arrangement of the invention, a radio frequency transmitter 258 near the surface of the earth i6 is made to radiate radio energy pulses at a pulse rate preferably such that the resulting pulse train for each transmitted pulse appearing at receiver 269 (illustrated in Fig. 1) due to the various modes of propagation will not overlap succeeding transmitted pulses. A repetition period of .05 second Was used in the present embodiment but other rates may' be equally suitable. Thus, for example, the paths 296 and 219V determined by the E ionosphere layer and the path 263 determined by the F ionosphere layer are each of a diierent length and therefore will cause a pulse from transmitter 258 to appear at receiver 2 69 as a pulseV train. 'Ihe receiver 269, being of the type described in Fig. 1, has its synchronizing pulse timer 254 adjusted by the manual control 256 sofas to produce negative trigger pulses |98 at the same rate as the pulses transmitted from transmitter 258. By further adjustment of adjusting arms 229 and 259 rFig. l), the gating pulse |94 may be adjusted so that a selected pulse or chain of pulses in the above train may thereby be singled out for angular information continuously recorded on recorder i555.

This invention is not limited to the particular details of construction and processes described, as many equivalents Will suggest themselves to those skilled in the art. It is accordingly desired that the appended claims be given a. broad interpreta-tion commensurate with the scope ofthe invention Within the art.

What is claimed is:

1. A system for measuring the angle of arrival at a receiving station of radio energy originating at a remotely located transmitting station, .comprising: means at said transmitting station for generating and radiating a pulse of radio energy; directionally sensitive means at said receiving station for intercepting said pulse of radio energy andv deriving therefrom pairs of composite sig'- nals, each signal constituting a. combination of a portion of said puise of radio energy reflected from the ionosphere and a portion thereof reflected from the earth; the signals of each pair of said composite signals diiiering from each other in amplitude asa function of the angles of arrival ,of those portions thereof reected from the ionosphere; means receptive of said pairs of composite signals for deriving from each pair two additional signals corresponding, respectively, to` the sum of and the difference between the amplitudes of the signals of each pair thereof; means at said receiving station, synchronized with the ,radiation of Asaid pulse of radio energy, for isolating the aforementioned two additional signals derived froing'the pair of said composite signals .correspending to ay selected mode of propagation' of said pulse of radio energy; and means receptive of said vlast-named two additional signals for comparing the same to determine the ratio of the amplitudes thereof.

2. A system for the angle of arrival at a receiving station of lradio energy originating at a remotely located transmitting station, comprisi-ng; means at said transmitting station for generating and radiating a pulseof radio energy;

directionally sensitive meansV at said receiving station, including a triple antenna array, for intercepting said pulse of radio energy and deri ing therefrom pairs of composite signals, each signal constituting a combination of a portion of said'pulse of radio energy reflected from the ionosphere afi-'d a portion thereof reiiected from the earth; the Vsignals of each pair of said composite signals differing from each other in amplitude as a function of the angles of arrival of those portions thereof reiiected from the ionosphere; means receptive 0f said pairs of composite signals for deriving from each pair two additional signals corresponding, respectively, to the sum of and the difference between the amplitudes of the signals of each pair thereof; means at said receiving station, synchronized with the radiation of said pulse of radio energy, forisolating the aforementioned two additional signals derived from the pair of said composite signals corresponding to a selected mode of propagation of said pulse of Iradio energy; and means receptive of said last-named two additional signals for comparing the same to determine the ratio of the amplitudes thereof.

3. A system for measuring the angle of arrival at a receiving station of radio energy originating at a remotely located transmitting station, comprising: means at said transmitting station for generating and radiating a pulse of radio energy; directionally sensitive means at said receiving station for intercepting said pulse of radio energy and deriving therefrom pairs of composite signals, each signal constituting a combination of a portion of said pulse of radio energy reflected from the ionosphere and a portion thereof reilected from the earth; the signals of each pair of said composite signals differing from each other in amplitude as a function of the angles of arrival of those portions thereof reflected from the ionosphere; means receptive of said pairs of composite signals for deriving from each pair two additional signals corresponding, respectively, to the sum of and the difference between the amplitudes of the signals of each pair thereof; means receptive of said last-named two additional signals for comparing the same to determine the ratio of the amplitude thereof; and means at said receiving station, synchronized with the radiation of said pulse of said radio energy, for controlling the point of time and the duration of the period of response of said lastnamed means to isolate the aforementioned two additional signals derived from the pair of said composite signals corresponding to a selected mode of propagation of said pulse of radio energy.

4. A system for measuring the angle of arrival at a receiving station of radio energy originating at a remotely located transmitting station, comprising: means at said transmitting station for generating and radiating a pulse of radio energy; directionally sensitive means at said receiving station for intercepting said pulse of radio energy and deriving therefrom pairs of composite signals, each signal constituting a combination of a portion of said pulse of radio energy reflected from ro the ion osphere and a portion thereof reflected from the earth; the signals of each pair of said vco'umposite "signals differingfrom each other Ain amplitudefas a function of the angles of arrival ofthoseportions?thereof'reflected from the ionospheregAmeans receptive of saidipairs of com- 'posite Ysignals for deriving from each pair two additional v'signals Y corresponding, respectively, to the ysuxn'of and the difference between the amplitudes ofthe signals of each pair thereof; means at said receiving station, synchronized with the radiationofwsaid pulsewof radio energy, yfor iqlatilsihe afremeriiioned iii/.0 addiiignal sisnals derived from the pair of said compositefsignals corresponding to a selected mode of propagation of said pulse of radio energy; and logarithmicallyresponsive means receptive of said last-named two additional signals for comparing the same to determine the ratio of the amplitudes thereof.

5. A system for measuring the angle of arrival at a receiving station of radio energy originating at a remotely located transmitting station, comprising: means at said transmitting station for generating and radiating a pulse of radio energy; directionally sensitive means at said receiving station, including a triple antenna array in a plane perpendicular to the earth, having the two outer antennas connected in parallel in one circuit and the center antenna connected in a second circuit, for intercepting said pulse of radio energy and deriving therefrom pairs of composite signals, each signal constituting a combination of a portion of said pulse of radio energy reflected from the ionosphere and a portion thereof reflected from the earth; the signals of each pair of said composite signals differing from each other in amplitude as a function of the angles of arrival of those portions thereof relected from the ionosphere; means receptive of said pairs of composite signals for deriving from each pair two additional signals corresponding, respectively, to the sum of and the defference between the amplitudes of the signals of each pair thereof; means at said receiving station, synchronized with the radiation of said pulse of radio energy, for isolating the aforementioned two additional signals derived from a pair of said composite signals corresponding to a selected mode of propagation of said pulse of radio energy; and means receptive of said last-named two additional signals for comparing the same to determine the ratio of the amplitudes thereof.

6. A system for measuring the angle of arrival at a receiving station of radio energy originating at a remotely located transmitting station, comprising: means at said transmitting station for generating and radiating a pulse of radio energy; directionally sensitive means at said receiving station, including a triple antenna array in a plane perpendicular to the earth, having the two outer antennas connected in parallel in one circuit and the center antenna connected in a second circuit, for intercepting said pulse of radio energy and deriving therefrom pairs of composite signals, each signal lconstituting a combination of a portion of said pulse of radio energy reflected from the ionosphere and a portion thereof reected from the earth; the signals of each pair of said composite signals differing from each other' in amplitude as a function of the angles of arrival of those portions thereof reiiected from the ionosphere; means receptive of said pairs of composite signals for deriving from each pair two' 1 1 additional signals corresponding, respectively, to the sum of and the diierence between'the amplitudes of the signals of each pair thereof ;Y logarthnzically-responsive means receptive of said last-named tvvoy additional signals for comparing the same to determine the ratio of the amplitudes thereof; and means at said receiving station, synchronzed with the radiation of said pulse of radio energy, for controlling the point of time and the duration of the period of response of said last-named means to isolatethe aforementioned two additional signals derived from the pairof A said composite signalscorresponding Ato` fa se'- lected mode of propagationof said pulse of radio energy. Y 1 Y Y JOSEPH BETrENcoURT.

12 REFERENCES CITED The following references are of record in the le of this patent:

UNITED lSTATES PATENTS Dippy June 27, 1950 4 

